Within the field of optometry, there exist many devices that are used to assess the direction of fixation of an eye. An example of such a device is described in U.S. Pat. No. 6,027,216, the contents of which are hereby incorporated in their entirety. Many such devices utilize a scanning laser beam to perform measurements.
One example of such a device is shown in FIG. 1. The device includes a light source 101, a beam splitter 102, a polarization beam splitter 103, photodetectors 104A and 104B, a motor 105 having a rotatable shaft, a first concave mirror 106, and a second concave mirror 107.
The light source 101 provides a diverging beam of polarized light which passes through beam splitter 102 and is incident on the first concave mirror 106. The first concave mirror 106 is mounted in a tilted fashion on the shaft of the motor 105 such that the first concave mirror wobbles 106 slightly when the shaft rotates. The first concave mirror 106 forms an image of the light source 101 on the surface of the second concave mirror 107. The second concave mirror 107 is stationary and is larger than the first concave mirror 106. As the shaft of motor 105 rotates, the image of the light source 101 on the surface of second concave mirror 107 is continuously scanned about a circular path. The curvature of stationary second concave mirror 107 can be chosen such that an image reflected from the spinning first concave mirror 106 is formed directly at the eye 108. All the light leaving the spinning first concave mirror 106 is imaged by stationary second concave mirror 107 to pass through a stationary exit pupil of the device, designated by the dashed circle, which overfills the pupil of the eye 108. The eye 108 sees the spinning image of the light source 101 in the form of a circle of light on the surface of stationary second concave mirror 107. A continuous annular scan of retinal areas is thus achieved by the light incident on the eye 108.
In order to allow for rapid measurements of the light reflected from the fundus, it is desirable to operate the above-described scanning at a scanning rate of at least 100 Hz and preferably at rates of 200 Hz or more. Scan rates at 200 Hz or more permit measurements to be obtained when working with subjects that may be less than fully cooperative, as is commonly the case with very young children. Such rates require the mechanical rotation of the first concave mirror 106 at rates which place special requirements on the mounting of the first concave mirror 106 and the motor 105 that spins it. In the case of retinal birefringent scanning, the first concave mirror 106 is tilted at an angle of approximately 1.5 degrees (to generate a tilt of approximately 3 degrees), and the first concave mirror 106 is then rotated about the axis of the chief ray of the optical beam.
Unfortunately, the tilt of the first concave mirror 106 can create a problem when it is rotated at high rates. Although the first concave mirror 106 is mechanically balanced when not rotating, the introduction of spin generates forces on the first concave mirror 106 (and the mechanical apparatus holding the mirror) that are not balanced, resulting in vibrations.
One known approach to minimize excessive vibration with a rotating tilted disk is to use a symmetrical disk which is of the same mass, size and shape of the tilted disk, but angled opposite to the angle of the tilted disk.
There are still potential shortcomings with this approach. Most notable is that the mass of the rotating object has doubled. For a device that performs scanning, this places extra time delay between the time when the motor is started and the time when the needed rotational speed has been achieved. This can make the device unsuitable for stopping and starting, and may require that the device is simply left with the motor spinning so that it is ready to use. Another potential shortcoming with this approach is that the tilted disk may have a shape that is not a simple flat disk but rather a concave disk such as the first concave mirror of the '216 patent. In this situation, a symmetrical concave mirror could be tilted at precisely the same angle (but in an opposite direction) as the first concave mirror. However, the additional component and the additional steps needed to fabricate this arrangement would result in a higher cost for the device. Additionally, there is a lack of machinery which is optimized for fabricating such assemblies and therefore the symmetrical disk approach can involve extra time in manufacturing in addition to the extra materials.
Another potential shortcoming with the symmetrical disk approach is that it can also be complex to resolve or correct for residual errors in manufacturing, which are virtually unavoidable for such an arrangement. Such errors generate vibrations, which need to be corrected. These types of errors are inherently difficult to correct because the assembly needs to be stopped in order to be adjusted, but the motor must be spinning in order to observe the vibration. Furthermore, making the necessary adjustments can be very time consuming.
As discussed above, methods of scanning a laser beam to perform measurements typically involve the mechanical movement of an optical device. For retinal birefringence scanning, there is a mirror that is both tilted and spinning at a high speed (e.g. 12,000 rpm). When utilizing mechanical movements of optical devices, vibrations can present significant complexities to scanning instruments. The vibrations must be kept low enough so as not to impact the measurements intended by the instrument.
There are other complications with using mechanical movements for scanning optical instruments, such as:
Lifetime of the assembly—the useful lifetime of the instrument is often limited by the life span of the motor, which has a shorter life span than virtually every other component of the optical scanning instrument.
Fabrication/Assembly—Regarding the fabrication/assembly of the instrument, the process is not likely to be automated. Rather, highly skilled personnel are likely needed to assemble the components to the tight tolerances needed to achieve the necessary balance and to make adjustments to minimize vibrations—all which lead to higher than optimal costs.
Noise—even relatively quiet motors will make an audible sound that can be distracting to a patient.
Safety/Durability—From a robustness perspective, any time a component (e.g., the motor, the shaft and/or the mirror) is spinning at such a high speed (such as 12,000 rpm), the component is more susceptible to failure (e.g., due to fatigue) and such failure can potentially cause significant damage to the instrument.
Cost—the combination of the above issues generates significant requirements on the design of the instrument that add time and materials to the production process, increasing overall cost.